1. Field of the Invention
The invention relates to an apparatus and method for applying powder to a workpiece. More specifically, the invention concerns a method and apparatus for applying an internal powder coating to metal container bodies such as an aluminum can body.
2. Description of the Prior Art
Powder internal coating of can bodies is a relatively new concept combining the technology of the can handling and spray coating arts. Particularly, the handling of aluminum cans presents numerous problems because of the thin can walls that are easily deformed in the process of shaping the can body, printing a label on the exterior, necking the open top end of the can body, and otherwise transporting the aluminum can body on conveyor means between processing stations. The present invention overcomes handling problems in loading the can bodies into the powder coating machine, placing the loaded can bodies into position to be internally coated, moving the can bodies out of coating position, and unloading the can bodies.
Internal coating of cans with powder has presented problems because prior art powder coating guns have often required strong external electric fields to guide the powder to the object to be coated. The "Faraday Cage" effect prevents powder from penetrating recesses of coated objects and accordingly prevents even distribution of powder in the interior of the can body. This problem is partially overcome through the use of electro-gas-dynamic (EGD) powder coating guns such as is disclosed in U.S. Pat. No. 3,853,580 to Gourdine. In addition, research suggests that tribo-electro-gas-dynamic (TEGD) powder coating guns will produce superior results with the present invention. EGD guns typically utilize a dielectric barrel while TEGD guns may have either a dielectric or conductive barrel. Either type of gun may be grounded.
Since the powder guns used with this machine deliver a spray of powder entrained in a propellant gas, means are needed to control the powder that does not remain attached to the object to be coated. In addition, known powder guns operate most effectively when in continuous operation and can spray a substantial amount of powder between workpieces. Means are needed to protect the mechanical parts of the powder coating machine from such overspray.
Prior powder coating technology teaches that an object to be powder coated must be electrically grounded, but some container bodies have nonconductive exterior coatings that make grounding difficult. In addition, equipment for handling aluminum can bodies may require a padded surface, usually constructed of a nonconductive resilient material, to avoid damage to the can bodies. Accordingly, the present invention overcomes the prior art requirement of grounding and allows the use of needed nonconductive resilient surfaces on the can handling equipment and also allows the internal coating of containers after a nonconductive exterior coating has been applied.